Not sure if this was posted before, but there's this site that posted a ISO 100-12800 comparison between Sony A55/Nikon D3100/Canon 60D
http://www.focus-numerique.com/test-1129/reflex-nikon-d3100-bruit-electronique-12.html
Finally another High ISO 12800 test has posted.
At ISO 12800, the picture looks terrible for all brands.
But refer at ISO 6400, Nikon D3100 looks performing better compared with Sony A55 and Canon 60D.
This is good !